Quote Originally Posted by JohnT
There are plastic boxes available that would think would be better. Don't use anything you don't want in your pool. The blocks could be pulled in if the water level drops or something heavy blows onto the cover.

http://www.intheswim.com/Winter-Pool...e-Aqua-Blocks/
And I'm here to tell you how true it is. I started my season this year with a cold swim in the deep end to fish out 10 8x12 inch pavers that had gotten pulled in when the wind came up and off went the cover. Also got to try my patching skills on two small (thankfully) holes in the liner.

What'd I know, that's how the pool came when we bought the house.

C.